In the usual buckle folding mechanisms with two folding pockets the folding pockets are fixedly installed in the paper feed direction. Two arrangements of the folding pockets are possible, namely with the first folding pocket, in the paper feed direction, below and the second folding pocket above or vice versa. There are different folding possibilities, depending on whether the first folding pocket is below or above. If such folding mechanisms with two folding pockets are fitted on envelope stuffing machines or similar machines, the form must have a specific position after leaving the folding mechanism. If for example the envelope stuffing machine positions the envelope with the window lying upwards, the form must be so folded that the address is on top following the folding mechanism, since an additional device would otherwise be necessary to turn the form over. In order to overcome this problem it would indeed be possible to use a buckle folding mechanism which has one folding pocket and one pair of folding rollers more than folds are needed in the form in question, i.e. a buckle folding mechanism with three folding pockets for folding a form with two folds and a buckle folding mechanism with four folding pockets for folding a form with three folds. One of the folding pockets can be switched out of action in each case by suitable deflectors and the form be so folded that the address lies selectively on top or underneath following the folding mechanism. However additional expense and sources of error arise from the greater number of folding pockets, and also cycle time losses through the increase in the path of travel.
In one known buckle folding mechanism of the kind referred to initially (DE 2 459 294 C2, FIG. 3) the folding mechanism block consists of a driving block and a folding mechanism head, with the folding rollers and the folding pockets, fitted thereon and attached interchangeably. The sheet inlet and the sheet outlet are arranged at different heights in the folding mechanism head, as are the inlet and the outlet planes. The folding mechanism head can be interchanged with other folding mechanism heads with differing constructions and differing layouts, so that it would here also be possible to replace a folding mechanism head whose first folding pocket lay below with another with a first folding pocket lying above. However two different folding mechanism heads would be needed in this case, whereby the manufacturing costs and servicing costs of the buckle folding mechanism increase and space has to be provided to store the unused folding mechanism head.
A paper folding machine is further known (DE-PS 517 549) in which one of the folding rollers is movably mounted, so that it can cooperate selectively with two different other folding rollers and in which one of the folding pockets can moreover be changed from one side of the machine to the other. It is indeed possible to produce oppositely directed folds selectively through this but the sheet outlet lies either on the inlet side of the sheet or on the side opposite the inlet side, depending on the position of the movable folding roller and the interchangeable folding pocket. However the result of this is that the transport device with which the folded sheet is transported away must be arranged on different sides of the buckle folding mechanism, depending on the folding direction, which would in practice require substantial setting up work or would actually not be possible, because the various operating and feed assemblies which follow the buckle folding mechanism cannot be brought simply out of their original arrangement into one directed the other way.